Conventional operational amplifiers are known to employ several transistor gain stages to establish high open-loop voltage gain. In order to achieve high gain in a differential gain stage, conventional, cascoding is often used to provide good D.C. balance and higher output impedance. One disadvantage of direct-coupled differential cascoded gain stages is that the bias circuitry increases the complexity of the gain stage. In addition, the signal current passes through all transistors in the circuit with concomitant time delays which contribute to instability when feed-back is included around the circuit.
In accordance with the preferred embodiment of the present invention, an improved differential gain stage achieves the benefits of a cascode configuration without causing signal current to pass through an additional transistor. The present circuit provides DC balance and local positive feedback for high gain, wide bandwidth operation.